Download and resize images explicitly

[csmith]

At the minute for inline images we just lob an Image node in the graph and let JavaFX sort it out.

We should really be downloading and resizing the image first, and if both of those steps succeed /then/ show it. We could maybe show a placeholder initially and edit it afterwards.

This will make it nicer if someone links to a *.png/gif/jpg/etc url that's not actually an image, and should improve the quality of the resizing which is pretty dire.